This invention relates to a process for producing silicon nitride. More particularly it relates to a process for producing silicon nitride of high purity which does not require further purification.
Silicon nitride is a refractory fibrous material. Depending on the length of the fibers and how they are processed, the material can be used in cutting tools, engines, reinforcing cast metal parts or by using the longer fibers, it can be made into blankets, felts for furnace walls and roof insulation, and flame curtains.
There are several known methods for producing silicon nitride, such as:
1. The reaction between elemental silicon and nitrogen;
2. The reduction of SiO.sub.2 with carbon in the presence of nitrogen; and
3. The reaction between SiCl.sub.4 and anhydrous NH.sub.3. Reactions 1 and 2 are actually the same. In reaction 2 the carbon first reduces the silicon dioxide to silicon which then reacts with the nitrogen to produce silicon nitride, or it may react with the silicon to produce silicon carbide which results in a decrease in the yield of silicon nitride.
A disadvantage of reaction 1 is that pure silicon is expensive and it also requires grinding to powder before it will react with the nitrogen. Grinding introduces impurities, making the process impractical if a pure product is required.
In 3, the reaction between SiCl.sub.4 and NH.sub.3 produces products which are both difficult and expensive to remove.
Probable steps of the reaction are: EQU Step 1 SiCl.sub.4 +8NH.sub.3 .fwdarw.Si(NH.sub.2).sub.4 +4NH.sub.4 Cl EQU Step 2 Si(NH.sub.2).sub.4 .fwdarw.Si(NH.sub.2).sub.3 NH.fwdarw.Si(NH).sub.2 EQU Step 3 Si(NH).sub.2 .fwdarw.Si.sub.3 N.sub.4 +NH.sub.3.
In step 1, the ammonium chloride must be sublimed off without losing the silicon nitride compound. This is difficult to do because the silicon nitride is so fine that it is carried out with the ammonium chloride. In step 3 the silicon diamide is converted to silicon nitride by heating at about 1400.degree. C. Of course any oxygen present causes oxidation back to SiO.sub.2, which makes the process difficult to control.
U.S. Pat. No. 3,244,480 discloses a process for producing silicon nitride which involves using inorganic materials as sources for the silicon dioxide. A disadvantage of using inorganic sources is that the silicon nitride can become contaminated with the residual decomposition products of the silicon dioxide source.